As is known in the field, a further development of the wideband code division multiple access (WCDMA)/universal mobile telecommunications system (UMTS) communication system is the definition of the system known as high speed downlink packet access (HSDPA). HSDPA operates as a time-shared communications channel which provides the potential for high peak data rates as well as the possibility for having a high spectral efficiency. HSDPA improves system capacity and increases user data rates in the downlink, that is, for transmission of data from a radio base station (BTS) to a user equipment (UE). BTS is also known as a Node B server in a UMTS system.
FIG. 1 illustrates a prior art radio interface protocol structure of HSDPA. It shows the relationship between the different layers among UE, Node B and various radio network controllers (RNCs). In FIG. 1, RLC is the radio link control layer, and MAC is the medium access control layer and PHY is the physical layer. MAC-hs (MAC high-speed) is a new MAC entity terminated in Node B for controlling the HS-DSCH transport layer. MAC-c (common MAC) is an entity in the UE, which transfers MAC-c PDU (protocol data unit) to the peer MAC-c entity in the RNC using the services of the physical layer. MAC-c/sh is responsible for the PCH (Paging Channel), FACH (Forward Access Channel), DSCH (Downlink Shared Channel) and RACH (Random Access Channel). MAC-d (dedicated MAC) is responsible for dedicated channels (DCHs) and is retained in the serving RNC, whereas MAC-c/sh is in the controlling RNC. L1, L1, L2 are radio resources for the radio resource controller connection. L1 is a physical layer and L2 is a data link layer.
An HS-DSCH channel is a downlink transport channel shared by several UEs. The HS-DSCH is associated with one downlink DPCH (downlink dedicated physical channel) or F-DPCH per active user, and one or several shared control channels (HS-SCCH). The HS-DSCH can be transmitted over the entire cell or over only part of the cell using beam-forming antennas, for example.
In terms of channels, there are three types of UMTS channel levels in a UMTS system so as to allow a UE to communicate with other network components: physical channels, transport channels and logical channels. The logical channels provide transport bears for information exchange between MAC protocol and RLC protocol. Transport channels provide the bearers for information between MAC protocol and the physical layer. Physical channels, which are identified by frequencies, spreading codes, etc., provide the transport bearers for different transport channels.
The logical channel can be used for communicating a PDU to or from a UE in a radio access network. Among various fields in the PDU, one is used to identify the UE (UE-id) and one is used to indicate the UE-id type. Most of the control signaling between UE and UTRAN is Radio Resource Control (RRC) messages. When the serving radio network controller (S-RNC) establishes the radio resource control (RRC) connection to a UE and decides to use a dedicated channel for this particular RRC connection, it allocates a UTRAN radio network temporary identity (RNTI) and radio resources L1, L2 for the RRC connection. An RRC connection set up message is sent from the S-RNC to the UE. It is known that UE has two basic operation modes, the Idle Mode and Connection Mode. The transition from the Idle Mode to the UTRAN Connection Mode is initiated by the UE by transmitting a request for RRC connection. When the UE receives a message from the network confirming the establishment of the RRC connection, the UE enters the CELL_FACH (forward access channel) state or CELL_DCH (dedicated channel) state of the UTRAN Connection Mode.
Although HSDPA is an efficient method for delivering relatively large amounts of data in relatively small time periods (the transmission time interval, or TTI, for a HSDPA system is 2 ms). This performance, however, can only be used when the user equipment is operating within the dedicated channel state (CELL_DCH state). In other words, the performance can be carried out only after a physical layer connection between UE and the BTS has been established and the layer connection has dedicated channels allocated to it. The transition from the UE Idle state to the dedicated channel state (CELL_DCH state) and establishing an HSDPA connection may take up to a second. When the amount of data required to be transmitted is relatively small, the state transition to the CELL_DCH state can take longer than the actual data transmission.
Moreover, when the UE is in the process of changing states to the CELL_DCH state, the required state change has to be addressed to the UE by the forward access channel (FACH). This required state change is significantly slower and less robust than the later HSDPA transmission channels. Before and during the transition to the CELL_DCH state, the CELL_FACH state requires that both the downlink dedicated control channel (DCCH) and the downlink dedicated traffic channel (DTCH) are mapped onto the forward access channel (FACH).
To avoid data loss during state transition between CELL_FACH state and CELL-DCH state, even when HSDPA MiMo (multiple-input multiple-output for transmit/receive diversity) is used, it is possible to stop downlink data transmission for certain time as the network is not aware of the exact time instance when UE is able to operate in CELL_DCH state and receive a correct downlink channel and the PDU format. In particular, it is possible to directly map MAC-d PDUs to MAC-c PDU as defined in Rel99 and then to map MAC-c PDU to MAC-hs PDU as shown in FIG. 2a (multiplexing structure) and FIG. 2b (PDU header structure).
In a non-HS-DSCH channel, a MAC PDU has a MAC header section and a MAC SDU (Service Data Unit) section. The MAC header section has four fields: a Coding of Target Channel Type Field (TCTF), a UE-id Type field, a UE-id (UE identity) field and a C/T field. The C/T field is used to provide identification of the logical channel instance when multiple logical channels are carried on the same transport channel. The TCTF field is used to provide identification of the logical channel class on FACH and RACH transport channels. The C/T field is also used to provide identification of the logical channel type on dedicated transport channels on FACH and RACH when used for user data transmission.
In a MAC-hs PDU consists of one MAC-hs header and one or more MAC-hs SDUs (Service Data Units). A maximum of one MAC-hs PDU can be transmitted in a TTI per UE. The MAC-hs header is of variable size. The MAC-hs PDU in one TTI (Transmission Time Interval) belongs to the same reordering queue. TTI indicates how often data arrives from higher layers to the physical layer. As shown in FIG. 2b, the MAC-hs header includes a priority Queue ID to identify a priority level of the MAC-d flow, a transmission sequence number (TSN) and one or more groups of three fields (SID, N and F), wherein SID (Size Index) indicates the length of each SDU, N indicates the number of SDUs having the length of the SID, and F (Flag) indicates whether the next field contains the SID length information. Thus, the group or groups of SID, N and F are indicative of the number or size of one or more subsequent protocol data units. Queue ID is also indicative of the reordering queue in the receiver. In addition, in front of the MAC-hs header, a version flag (VF) is also provided.
In the above-described multiplexing scheme using the PDU structure, the transition from the UE to the dedicated channel state (CELL_DCH state) and establishing an HSDPA connection may take more time than the actual data transmission, especially when the amount of data required to be transmitted is relatively small. The PDU structure as shown above can be further improved.